Fighting an infection almost always raises your heart rate, often noticeably. A febrile adult’s heart rate averages roughly 8 to 10 beats per minute faster for every degree Celsius of fever, and the effect can be even larger in young children. But fever is only part of the story. Your nervous system, your immune signaling molecules, and even your hydration status all push the heart to beat faster during illness, sometimes before you realize you are sick at all.
How Fever Pushes Your Heart Rate Up
The most straightforward reason your heart beats faster during an infection is that your body temperature rises. A higher core temperature speeds up the chemical reactions inside cells, increases oxygen demand in tissues, and prompts blood vessels near the skin to dilate so heat can escape. To keep up with all of that, the heart pumps more frequently. In a study of hospitalized adults recovering from febrile illness, heart rate during fever averaged about 84 beats per minute compared with roughly 67 beats per minute after recovery, and each 1 °C rise in temperature corresponded to an average increase of about 8.5 beats per minute.1PubMed. Fever and cardiac rhythm
In children the effect is even more pronounced. A large study of children under 16 seen in urgent and emergency care found that heart rate rose by an average of about 12 beats per minute for each 1 °C of fever, with the youngest children showing increases as high as roughly 14 beats per minute per degree and older children closer to 9.2PubMed Central. The association between temperature, heart rate, and respiratory rate in children aged under 16 years attending urgent and emergency care settings These numbers help clinicians figure out whether a child’s fast heart rate is simply proportional to their fever or whether something else, like dehydration or a more serious infection, is driving it higher than expected.
The Nervous System Behind the Acceleration
Fever accounts for a lot of the heart-rate increase, but your sympathetic nervous system, the branch responsible for the “fight or flight” response, adds its own contribution. When immune cells detect an invader, they release signaling molecules called cytokines. Those cytokines communicate with the brain, which in turn ramps up sympathetic nerve activity throughout the body. The result is higher levels of stress hormones like adrenaline and noradrenaline, both of which directly speed up the heart.
Research over several decades has shown that the sympathetic nervous system plays a central role in regulating the inflammatory response itself. It helps redirect energy-rich fuels like glucose and amino acids toward the activated immune system, essentially putting the body into a high-output metabolic state.3PubMed Central. The sympathetic nervous response in inflammation That metabolic overdrive is one reason you feel exhausted during a bad infection: your body is burning through resources at a faster clip, and your heart is working harder to deliver them. If the inflammatory state becomes chronic, as it can in autoimmune conditions, the sustained sympathetic activation can contribute to high blood pressure and insulin resistance over time.3PubMed Central. The sympathetic nervous response in inflammation
Psychological stress can amplify this cascade. When someone is already stressed and then encounters an immune challenge, the inflammatory cytokine response tends to be larger, and blood pressure and stress hormone levels climb higher than either stressor would produce on its own.4PubMed Central. Synergistic effects of psychological and immune stressors on inflammatory cytokine and sickness responses in humans In practical terms, if you are sleep-deprived or under heavy work pressure when you catch the flu, the cardiovascular effects of that illness may feel more intense.
What Heart Rate Variability Reveals
Heart rate variability, the slight beat-to-beat fluctuation in the time between heartbeats, tells a different but complementary story. In a healthy person at rest, the heart does not beat like a metronome; it speeds up and slows down subtly with each breath and with changes in posture, emotion, and autonomic tone. Higher variability generally reflects a well-balanced nervous system. During infection, that variability drops.
A systematic review examining heart rate variability across inflammatory states found that a pro-inflammatory state was consistently linked to reduced variability, including reductions in both the vagal (parasympathetic) and non-vagal components of the signal.5PubMed. Heart rate variability as a marker and predictor of inflammation, nosocomial infection, and sepsis – A systematic review In other words, when your immune system is actively battling something, the nervous system’s normal push-and-pull over heart rhythm becomes lopsided, with the sympathetic “go faster” side dominating.
The drop in variability can sometimes appear before other clinical signs of infection worsen. In a small study of hospitalized COVID-19 patients, ten out of twelve who experienced a large spike in an inflammatory blood marker showed a notable drop in heart rate variability in the 72 hours leading up to that spike.6Military Medicine. Heart Rate Variability as a Possible Predictive Marker for Acute Inflammatory Response in COVID-19 Patients That kind of early warning signal is one reason researchers are interested in continuous monitoring tools. However, experimental work in healthy volunteers given a bacterial toxin to trigger inflammation found that although heart rate variability dropped sharply, the size of the drop did not correlate neatly with how much inflammation each person developed.7PubMed Central. Interplay between the innate immune response and heart rate variability in healthy human volunteers The signal is useful for flagging that something is happening, but using it to gauge severity is still a work in progress.
Tachycardia as a Clinical Warning Sign
Clinicians have long treated a fast heart rate during infection as a red flag worth watching. The Systemic Inflammatory Response Syndrome criteria, or SIRS, were developed as an early screening tool for serious infection and sepsis. To meet the criteria a patient needs at least two of four signs: fever or abnormally low temperature, fast heart rate, rapid breathing, and an abnormal white blood cell count.8PubMed Central. The Systemic Inflammatory Response Syndrome (SIRS) in acutely hospitalised medical patients: a cohort study A heart rate above 90 beats per minute is the usual threshold. SIRS criteria have their limitations and have been supplemented by newer scoring systems, but the inclusion of tachycardia underscores how reliably infection speeds the heart.
In older adults suspected of sepsis, tachycardia and abnormal white blood cell counts were both risk factors for having bacteria in the bloodstream, though abnormal body temperature was the strongest single predictor.9PubMed Central. Systemic inflammatory response syndrome is more associated with bacteremia in elderly patients with suspected sepsis in emergency departments This matters because in an emergency department, checking heart rate is instantaneous and free, making it a practical triage tool even in resource-limited settings.
When Infection Does Not Raise Heart Rate
Not every infection follows the expected pattern. A phenomenon called relative bradycardia occurs when a patient has a high fever but a heart rate that is slower than it should be for that temperature. Normally you would expect an increase of roughly 8 to 10 beats per minute per degree Celsius of fever. When the pulse lags well behind that expectation, clinicians take note.
Certain intracellular, gram-negative bacteria are particularly associated with this paradox. A study comparing pulse-temperature relationships across a range of infections found that relative bradycardia was a statistically significant feature of typhoid fever, Legionnaires’ disease, and pneumonia caused by Chlamydia species, but not of other common pulmonary infections or typical gram-negative bloodstream infections.10PubMed. Relative bradycardia in infectious diseases The pattern appears limited enough that it is not helpful as a general diagnostic clue, but when a clinician suspects one of these specific pathogens, a surprisingly low heart rate can reinforce the suspicion.
The mechanism behind relative bradycardia is not fully pinned down. One hypothesis is that inflammatory cytokines directly affect the heart’s pacemaker cells, altering their responsiveness to the sympathetic signals that would normally speed things up. There may also be cross-talk between the autonomic nervous system and the immune system that, in certain infections, tilts the balance toward vagal dominance instead of the usual sympathetic overdrive.11PubMed. Proposed mechanisms of relative bradycardia
Age Changes the Heart Rate Response
Older adults tend to mount a blunted heart rate response to infection compared with younger people. A systematic review and meta-analysis of vital signs in adults presenting to hospitals with bacterial infections found that older adults were less likely to be tachycardic, with a mean heart rate about 5 beats per minute lower than younger patients with comparable infections. Looking across all ages studied, heart rate during bacterial infection declined progressively with age.12Oxford Academic. Is age associated with different vital signs in adults presenting to hospital with bacterial infection? A systematic review and meta-analysis
This is worth knowing because a “normal” heart rate in a sick 80-year-old does not mean the infection is mild. The aging heart and nervous system simply respond less vigorously to the same inflammatory signals. If clinicians relied on heart rate alone to triage severity, they might underestimate how sick an older patient really is. The same caution applies to people taking beta-blockers or certain other medications that limit how fast the heart can go regardless of what the immune system is doing.
Smartwatches and Early Detection
The connection between infection and elevated heart rate has given wearable technology an unexpected clinical application. Because fitness trackers and smartwatches measure resting heart rate continuously, they can spot the uptick that accompanies an infection before the wearer feels sick enough to notice.
A study during the early COVID-19 pandemic analyzed smartwatch data from a cohort of nearly 5,300 participants, 32 of whom were confirmed to have COVID-19. About 81% of the infected individuals showed detectable changes in heart rate, step count, or sleep patterns. Among cases with symptom information available, 22 out of 25 were flagged before or at symptom onset, and four were detected at least nine days before symptoms appeared. Using a two-tiered alert system based on extreme elevations in resting heart rate compared with each person’s baseline, the researchers estimated that about 63% of cases could have been caught pre-symptomatically in real time.13PubMed Central. Pre-symptomatic detection of COVID-19 from smartwatch data
A later study tested a more complex smartwatch algorithm that combined physiological parameters with symptom questionnaires for detecting pulmonary infections. When the algorithm used only physiological signals like heart rate and skin temperature, its sensitivity was modest at about 38%. Adding self-reported symptoms brought overall accuracy up to roughly 86%.14PubMed Central. Smartwatch-based algorithm for early detection of pulmonary infection: Validation and performance evaluation The takeaway is that wearable heart rate data is a useful signal but not definitive on its own; a slightly elevated resting heart rate for a few days could reflect an emerging infection, or it could reflect poor sleep, more caffeine, or a stressful week.
When a Fast Heart Rate Lingers After the Infection Clears
For most infections, heart rate returns to normal once the fever breaks and the immune response winds down. But some people, particularly after COVID-19, find that their heart rate stays uncomfortably fast for weeks or months. The most studied version of this is postural orthostatic tachycardia syndrome, or POTS, in which heart rate spikes excessively upon standing. Estimates suggest that somewhere between 2% and 14% of COVID-19 survivors develop POTS, and a much larger proportion, possibly between 9% and 61%, experience POTS-like symptoms such as tachycardia, fatigue, and difficulty concentrating within six to eight months of infection.15Heart Rhythm. Post–COVID-19 Tachycardia Syndrome: Epidemiology, Pathophysiology, and Management
POTS after infection is not unique to COVID-19. It has been reported after mononucleosis, influenza, and other viral illnesses, though the post-COVID wave made it far more visible. The working theory involves lingering autoimmune damage to the nerves that regulate blood vessel tone, leading the heart to compensate with an inappropriately high rate. For people who notice that their resting heart rate remains elevated long after a viral illness, this is worth discussing with a doctor rather than dismissing as lingering fatigue.
Myocarditis and Arrhythmias
In rare cases, the infection itself damages heart tissue directly. Viral myocarditis, an inflammation of the heart muscle, can occur when a virus infects cardiac cells. Several mechanisms are thought to contribute: the virus may directly lyse heart muscle cells and destabilize their electrical activity, it may cause small-vessel damage in the coronary circulation, and it may disrupt the gap junctions that coordinate the electrical signal between neighboring cells.16Heart Rhythm. Arrhythmias in myocarditis: State of the art Any of these can produce abnormal heart rhythms ranging from a mild racing sensation to dangerous arrhythmias.
Myocarditis is uncommon during a routine cold or flu, but it has been associated with a range of viruses including enteroviruses, adenoviruses, parvovirus B19, and SARS-CoV-2. The key practical point is that chest pain, unusual palpitations, or shortness of breath during or soon after an infection deserve medical attention, especially in someone who is young and otherwise healthy. Athletes are sometimes advised to avoid intense exercise for a period after a significant viral illness precisely because of this risk.
Fever Reducers Bring Heart Rate Down Too
If your heart rate climbs during a fever, it makes sense that treating the fever would bring the rate back down, and that is exactly what happens. A randomized trial of intravenous acetaminophen in febrile critically ill adults found that the acetaminophen group had a heart rate roughly 6 beats per minute lower than the placebo group in the first two hours after treatment, along with reductions in temperature and blood pressure.17PubMed. Effects of IV Acetaminophen on Core Body Temperature and Hemodynamic Responses in Febrile Critically Ill Adults: A Randomized Controlled Trial By four hours, however, the differences were no longer statistically significant, which aligns with anyone’s lived experience that the relief from a dose of acetaminophen or ibuprofen is temporary.
This raises an important nuance for monitoring. If you are using fever reducers and then checking your heart rate, you may get a falsely reassuring number. Your heart rate may look normal not because the infection is mild but because the medication has masked the fever-driven increase. Clinicians are well aware of this, but it can fool a person checking their own vitals at home.
Daily Rhythms Persist Even During Severe Illness
One intriguing finding from continuous monitoring of hospitalized COVID-19 patients is that the circadian pattern of heart rate, the natural dip at night and rise in the morning, persists even in severe illness. A study using wearable sensors found that rhythmic fluctuations in heart rate were present across all patient groups, whether they ultimately recovered, needed respiratory support, or died.18PLOS ONE. Circadian patterns of heart rate, respiratory rate and skin temperature in hospitalized COVID-19 patients However, the circadian rhythms of respiratory rate and skin temperature broke down in the sickest patients and were only preserved in those who recovered. The heart, in other words, keeps its internal clock ticking even when other body rhythms have gone haywire. Whether the loss of circadian patterning in respiratory rate could serve as an early prognostic marker is an open question, but the contrast with heart rate is striking and suggests that different organ systems lose their daily rhythmicity at different thresholds of illness severity.